Bird Flight

New research proves that birds in V formation arrange themselves in aerodynamically optimum positions

Ibises flying in formation

Ibises flying in formation (photo credit: Mark Unsöld)

Researchers at the RVC studied a free-flying flock of northern bald ibises by following them in a microlight, using specially developed GPS biologging technology to measure the position, speed and heading of all birds in a V formation, and when each bird flapped its wings.

This groundbreaking research, which appears on the front cover of the journal Nature, proves for the first time that birds flying in a distinctive V formation strategically position themselves in aerodynamically optimum positions, and experience positive aerodynamic interactions that maximise upwash (“good air”) capture.

This is achieved firstly through spatial phasing of wing beats when flying in a spanwise (‘V’) position, creating wing-tip path coherence between individuals to maximise upwash capture throughout the entire flap cycle. Secondly, when flying in a streamwise (‘behind’) position, birds exhibit spatial anti-phasing of their wing beats, creating no wing-tip path coherence and avoiding regions of detrimental downwash. Such a mechanism would be available specifically to flapping formation flight.

These aerodynamic accomplishments were previously not thought possible for birds because of the complex flight dynamics and sensory feedback that would be required to perform such a feat.

Portugal, S.J., Hubel, T.Y., Fritz, J., Heese, S., Trobe, D., Voelkl, B., Hailes, S., Wilson, A.M. & Usherwood, J.R. (2014). Upwash exploitation and downwash avoidance by flap phasing in ibis formation flight. Nature 505, 399-402. doi:10.1038/nature12939

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Leading edge vortices and force generation on a reductionist bird model

For my Ph.D I built an artificial, flapping goose model and investigated the forces and aerodynamics in a wind tunnel using Particle Image Velocimetry, a laser technology, that allows the visualization of the flow movement around and behind the wing.

igor

The goal was to gain a better understanding of the forces generated by the flapping wings, a knowledge essential to understand the necessary morphological requirements for flapping flight. I demonstrated that with a very simple flapping motion, enough force can be generated to support body weight, but thrust generation requires a more complex motion (Hubel and Tropea, 2009). In addition, using flight parameters (flapping frequency, speed, amplitude) characteristic for large birds, we found that slow flapping wings can generate a leading edge vortex, which had previously been assumed to be present in insect flight only. However, due to the low flapping frequency birds use, these flow structures are unstable and therefore potentially harmful, supporting the separation of the flow from the wing surface. One can conclude that adjustable wings (variable torsion) capable of stabilizing or preventing these leading edge vortices are therefore essential in bird flight (Hubel and Tropea, 2010).

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